JP2000115202A - Network connector and communication control method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a communication control method that loops a bus type network while maintaining the merit of the network, that is, high-speed communication. SOLUTION: A bus type network of a daisy chain system interconnects a plurality of nodes 10a-10d, and the network connector 20 interconnects both ends of the network. When the bus type network is normal, a switch 24 is open and data received via 1st and 2nd communication sections 21, 22 are aborted. On the other hand, when the bus type network is faulty (a broken cable 13 or the like), the switch is closed. Then data received by one communication section can be transferred to the other so as to loop the network thereby to surely transmit frames to all the nodes.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a network connection device and a communication control method, and more particularly, to a device for connecting both end points of a bus network.

[0002]

2. Description of the Related Art As is well known, a bus type network is one type of network. In this network, for example, as shown in FIG. 1, a plurality of nodes 1 are connected by a cable 3 via a built-in communication unit 2. At this time, each node 1 is connected to one or two adjacent nodes 1 so as to take a single series-connected structure as a whole. As a result, the nodes 1 located at both ends of the network are connected to another adjacent node, and the remaining nodes 1 are connected to two adjacent nodes 1.

The communication section 2 has a function of passing a signal (frame) transmitted from one cable 3 to the other cable 3 and a certain condition among the transmitted signals (for example, a destination addressed to itself). ) Has the ability to capture frames. Note that nodes located at both end points terminate without passing through. Because of this kind of through,
A frame output from each node connected to the network can be transmitted to the entire network at high speed.

[0004]

The above-mentioned bus network of the digital chain system has the following problems. In other words, when a failure occurs in the transmission system such as a break in a certain cable 3, the transmission of signals is interrupted at the break point, so that the frame cannot be sent to the entire network.

[0005] In order to solve this problem, it is preferable to form a loop (ring) network in which the network is formed into a loop. That is, even if one cable breaks, the frame can be sent to the entire network by wrapping around from the opposite side. However, in the case of this loop type network, each node
Since a frame can be transmitted only to an adjacent node, for example, when a frame is sent to a non-adjacent node, the intermediate nodes are sequentially relayed (unlike the through described above, the data is temporarily And then send it out). Therefore, high-speed communication cannot be performed unlike the bus network.

When a bus-type network is connected in a loop, frames are transmitted to the entire network, so that collision between frames occurs in each node and transmission may be disabled. Therefore, when constructing a conventional loop type network, there is no idea to use a bus type, and frames are sequentially relayed by nodes having a relay function as described above and transmitted to a target node.

If it is desired to stop the operation of all nodes at once, it is necessary to create and transmit a broadcast frame for stopping the nodes, for example. There is a time lag in stopping at the node. The same can be said for restarting after stopping.

SUMMARY OF THE INVENTION The present invention has been made in view of the above background, and has as its object to solve the above-mentioned problems and to form a loop (pseudo-network) while maintaining the advantage of high-speed communication of a bus-type network. An object of the present invention is to provide a network connection device and a communication control method that can perform looping. Another object of the present invention is to provide a network connection device and a communication control method capable of simultaneously stopping each node connected to the network in such a bus network.

[0009]

In order to achieve the above object, according to the present invention, both end points of a bus type network are usually terminated, but both end points are connected to a network connection device, respectively. By forming an upper loop and appropriately processing the received data in the network connection device, for example, a bus-type network capable of normal high-speed communication or an advantage of the loop-type network can be obtained. . In this regard, the claimed inventions are common. Claims 1 and 2 claim the invention by focusing on the configuration of a single network connection device, and claims 3 and 4 claim the invention by focusing on a communication control method when an actual network is constructed. Is complaining.

[0010] Specifically, the network connection device according to the present invention is a network connection device that can be connected to and disconnected from both ends of a bus-type network,
A first function of discarding data sent from the both end points, and a second function of transferring data sent from one end point of the bus network to the other end point,
Control means for switching between the first function and the second function to enable either one (in the embodiment, the information processing apparatus 2
3 (corresponding to claim 3). In the embodiment, the first function is to shut off the first and second communication units by opening the switch 24 (connecting the first and second changeover switches 24a and 24b to the other input terminal b). Is realized by: The second function is realized by connecting the first and second communication units by reversing the same and mutually transferring received data.

Another solution is to connect to both ends of a bus type network so that transmission and reception are possible.
Communication units (in the embodiment, the first and second communication units 21,
22), switch means for connecting the two communication units (corresponding to the switch 24, the first and second changeover switches 24a and 24b in the embodiment), and control means for controlling the switching operation of the switch means (Corresponding to the information processing device 23 in the embodiment), and by switching the switch means, a first state in which the two communication units are shut off and the two communication units are connected, and one of the communication units is connected. May be configured to be able to be in the second state in which the data received in (1) is transferred to the other communication unit and transmitted.

As another solution, there are two connection points that can be connected to and disconnected from both ends of the bus-type network (corresponding members are not shown in the embodiment, but each communication unit 21 and 22 (network connection device 20)
And a network connection device having a connection terminal provided on the network connection device for connecting the connection cable 14 and which is actually a connector terminal or a screw terminal. Determining means for determining whether the network is normal or abnormal (corresponding to the information processing apparatus 23 in the embodiment); and one of the two connection points according to the determination result of the determining means. Control means (corresponding to the information processing apparatus 23 in the embodiment) for switching between control for sending data taken from the server to the other connection point and control for not sending the data (claim 3). . In the embodiment, the first function is:
Open the switch 24 (first and second changeover switches 24a,
24b is connected to the other input terminal point b).
This is realized by shutting off the first and second communication units. The second function is realized by connecting the first and second communication units by reversing the above, and mutually transferring received data. Note that the transmission includes the case of direct through and the case of transfer performed via a buffer or the like.

In any one of the first to third aspects of the present invention,
When the first function is exhibited by the control means (in the first state, no transmission), no frame collision occurs on the network, and the configuration is equivalent to that of a normal bus-type network. Therefore, high-speed communication becomes possible. On the other hand, when the second function is exhibited by the control means (in the second state, sending), both end points (both connection points)
Are connected, so that frames (data) can be transmitted from one end point (connection point) to the other end point (connection point). Therefore, the network can be looped (pseudo-looped).

That is, even when a part of the bus type network is cut off or the like and transmission through that part is not possible, frames can be transmitted through the network connection device and the system is resistant to failure. Note that in the event of a failure in this way, strictly speaking, the nodes located at both ends of the failure have constructed and operated a bus network in which both ends are located. In this sense, it is called a quasi-loop (although it looks like a loop, it is not a loop-type network, and it is divided at a fault location).

In the first and second aspects of the present invention, a loop may be formed when the bus network is operating normally. In such a case, a frame collision occurs, so that network communication becomes impossible and each node becomes inoperable. Therefore, for example, when it is desired to stop all the nodes simultaneously, it can be effectively used.

On the other hand, the communication control method according to the present invention is a communication control method in a network in which both ends of a bus-type network are connected by a network connection device, wherein when the bus-type network is normal, the network connection device The received data sent into the network connection device is discarded, and when the bus type network is abnormal, the received data sent into the network connection device from one end is transferred to the other end (claim 4). .

Thus, in the normal state, the received data is discarded in the network connection device, so that frame collision does not occur in a node connected to the bus type network or the like. Therefore, normal communication of the bus network is performed. Even when the network is abnormal (transmission path: cable failure), frames can be transmitted via the network connection device (a pseudo loop is formed).
The transmission of frames over the entire network becomes possible.

A communication control method in a network in which both ends of a bus-type network are connected by a network connection device, wherein in a normal state, received data sent into the network connection device is discarded, The communication between the nodes connected to the network may be disabled by mutually transferring the received data sent into the network device (claim 5).

When a transfer is made in a state where the bus type network is operating normally, that is, when a loop is formed, a frame collision occurs. In this way, by actively colliding frames and disabling network communication, a simultaneous broadcast frame for stopping the node is created, and the nodes are simultaneously stopped (disabled) without sending the broadcast frame. Can be.

In each invention, discarding the received data means that the received data is not transferred, that is, the sent received data is not transmitted (transferred) from the network connection device to the network again. Therefore, the received data may or may not be used in the network connection device. When used, it can be used for determining the presence or absence of a network failure / abnormality, or for monitoring the state of the network (collecting information).

[0021]

FIG. 2 shows a preferred embodiment of a network connection device according to the present invention and a network in which the device is mounted. As shown in the figure, in this embodiment, four nodes 10a to 10d are connected by a cable 12 via a digital chain bus type network. The cable 12 is configured using an optical fiber.

As a system for constructing such a digital chain bus type network, for example, at least one
One node can be configured using a PLC. For example, for one master station composed of PLC, for example,
There is a type in which various components such as a plurality of remote I / O slave stations, transmission terminals, programmable displays, and inverters are connected and linked in a digital chain system using a cable composed of a two-core cable or an optical fiber. Further, each node is constituted by a PLC, and the PLC
The CPU units may be connected in a digital chain system using a cable such as an optical fiber to configure a system in which each PLC performs a certain process by performing distributed and cooperative control. Further, instead of connecting the CPU units to each other, an Ethernet unit may be provided in the PLC configuring each node, and the PLC units may be connected via the Ethernet unit on a ten-to-two basis. Of course, other network configurations can be adopted.

Each of the nodes 10a to 10d has a communication unit 13
Is built in, and the communication units 13 of adjacent nodes are connected by a cable 12. For convenience of illustration, the cable 12 is shown by a single line, but each has two built-in paths, up and down. In other words, for example, the cable 12 connecting the first node 10a and the second node 10b is connected from the first node 10a to the second node 10b.
A transmission path for transmission to the second node 10b to the first
Two transmission paths for transmitting to the node 10a are provided. Therefore, no frame collision occurs during transmission through the cable 12.

The communication section 13 has a function of passing through the transmitted frame as it is and transmitting it to the next node as described later.
Data transmitted by d can be broadcast to other nodes at once. Of course, if the passing frame is addressed to itself, it will be imported into the node,
A predetermined process corresponding to the content of the frame is performed.

Here, according to the present invention, one ends of connection cables 14 are connected to the communication units 13 in the first and fourth nodes 10a and 10d at both ends of the bus type network, respectively.
Connect the other end of the connection cable 14 to the network connection device 2
The connection is made to the first and second communication units 21 and 22 in 0. Actually, the connection cable 14 is connected to a connection point such as a connector or a screw terminal provided in the network connection device 20.
Connect. Then, the connection point and the first and second communication units 2
Since the nodes 1 and 22 are internally connected, the nodes 10a to 10d are apparently connected in a loop by the cable 12, the connection cable 14, and the network connection device 20.

The first and second communication units 21 and 22
The received frame is sent to the information processing unit (MPU) 23. Further, both communication units 21 and 22
Are connected via a normally open contact switch 24.
Therefore, when the switch 24 is open, the frame sent from the first node 10a is sent to the information processing unit 23 via the first communication unit 21 and discarded there. Similarly, the frame sent from the fourth node 10d is sent to the information processing unit 23 via the second communication unit 22, where it is discarded.

Therefore, in this state, since both end points constituting the network are not connected in the network connection device 20, the network becomes a normal digital chain bus type network, and each node does not collide. High-speed communication is possible with 10a to 10d.

On the other hand, when the switch 24 is closed, the frame transmitted from the first node 10a is transmitted to the first communication unit 21.
Through the switch 24 and the second communication unit 22
From the second communication unit 22 to the fourth node 10d. Similarly, the frame transmitted from the fourth node 10d reaches the first node 10a via the second communication unit 22 → the switch 24 → the first communication unit 21. Thereby, a loop type network is configured.

As described above, by opening and closing the switch 24, it is possible to switch between the bus type network and the loop type network. By adopting a necessary network configuration according to the situation, the advantage of the loop type while maintaining high speed communication is obtained. Can also be secured.

FIG. 3 shows an example of this use mode.
As shown in (1), when one cable 12 constituting the transmission path fails, communication between the third node 10c and the fourth node 10d connected via the cable 12 becomes impossible.
In this case, by closing the switch 24,
Communication between the third nodes 10a to 10c and the fourth node 10d can be performed using a transmission system passing through the connection cable 14 and the network connection device 20. When the failure of the cable 12 is recovered, the switch 2
Opening 4 restores the original normal bus network. As a result, without collision of frames
Communication between the nodes 10a to 10d can always be reliably performed.

The failure of the cable 12 includes not only a failure such as disconnection of the cable 12 itself, but also a failure of a connection portion between the cable 12 and the communication unit 13. In the present embodiment, since the cable 12 uses an optical fiber, each of the communication units 13, 21, 22 has an optical-electrical conversion function. Thereby, as described above, the cable 1
When the failure 2 occurs, the communication is automatically terminated at the communication units of the third and fourth nodes 10c and 10d connected to the failed cable 12. If the cable is an electrical communication medium such as a coaxial cable, a terminating device may be separately provided, and if a failure occurs as described above, the cable may be connected to the terminating device.

The opening / closing of the switch 24 is not limited to the case where a failure occurs as described above, but can be applied to, for example, an emergency stop of all the nodes 10a to 10d connected to the network due to some factor. . As an example, when each of the nodes 10a to 10d is configured by a control device such as a PLC and the PLC performs a predetermined process such as manufacturing and / or monitoring while performing a distributed and / or cooperative operation, the predetermined There are times when you want to stop processing.

In such a case, a conventional broadcast frame for stopping a node is created and transmitted. In this embodiment, the switch 24 is closed to close each node 10.
Frame collisions occur at a to 10d. As a result, each of the nodes 10a to 10d becomes inoperable, so that the operation is stopped. That is, while all the nodes are stopped only after the broadcast frame is received by all the nodes, it can be stopped simply by closing the switch 24 without creating or sending a stop command (frame). Also, when returning, only the switch 24 needs to be opened. Conventionally, when constructing a network, development has been performed on the premise that frame collision does not occur, but in the present embodiment, the idea is changed by 180 degrees and frame collision is actively generated (utilized). I did it.

Next, a specific configuration of each device for constructing the above-described system will be described. FIG.
It shows an example of the internal structure of a to 10d. Basically, each node has the same configuration, and as shown in the figure, is connected to two cables (optical fibers) 13 (in some cases, one is connected to a connection cable 14), and one communication for transmitting and receiving data. The communication unit 12 includes an MPU 15 that transmits and receives data to and from the communication unit 12.

The communication unit 12 includes two data transmission / reception units 12a and 12b connected to the cable 13, a data transmission unit 12c connected to the data transmission / reception units 12a and 12b and mutually transferring data, and a data transmission unit A data processing unit 12d interposed between the unit 12c and the MPU 15 for performing predetermined data conversion and the like. The specific functions and configurations of each unit are as follows.

The data transmission / reception units 12a and 12b are composed of optical-electrical conversion elements for ensuring compatibility between data (light) flowing on the cable 13 and data (electricity) flowing in the node.

As shown in the figure, the data transfer unit 12c receives the data transmitted from one data transmission / reception unit 12a.
The signals are provided to the two adders K1 and K2, transmitted to the data processing unit 12d via the adder K1, and transmitted to the other data transmission / reception unit 12b via the adder K2. Similarly, the data sent from the other data transmission / reception unit 12b is given to two adders K1 and K3,
Is transmitted to the data processing unit 12d via the
3 to one data transmission / reception unit 12a.
Further, the data sent from the data processing unit 12d is added to each data processing unit 12d via adders K3 and K2.
a, 12b.

That is, the communication unit 1 is connected via each cable 13.
2 is sent out to the cable 13 on the opposite side via the adder K3 or K2. That is, the data is transferred (through). Also, since the data is sent to the data processing unit 12d via the adder K1 at the same time, when the sent data is a frame addressed to itself, it can be taken into the inside, that is, into the MPU 15. When transmitting a frame from its own node to another node, the transmission data is transmitted from the data processing unit 12d via the two adders K3 and K2 to the two data transmission / reception units 12a and 12b.
(Cable 13).

Further, the data transfer section 12c shapes a signal when passing through the data transfer section 12c.
Thus, the through data can be transmitted to each node while retaining correct information without distortion.

The data processing section 12d has a function of converting a signal and a data framing function. That is,
The data flowing on the cable 13 is, for example, a continuous code sequence of “1/0”, and the data (signal) in the MPU 15 is a different code. Therefore, the code is converted (function of performing signal conversion). ).

The data flowing on the cable 13 is
As shown in FIG. 5, information such as a destination address, a self-address (source address), and an error check is added before data to be actually transmitted. Such information is unnecessary for processing in the MPU 15. Therefore,
The data transmission / reception unit 12
In the data (frames) sent from a and 12b, the transmission destination address is searched. In the case of the data addressed to itself, the information added before is removed, and only the actual data is extracted and sent to the MPU 15. Of course, those that are not addressed to themselves are discarded as they are. Similarly, necessary information is added before the actual data sent from the MPU 15 and is provided to the data transfer unit 12c. Note that MPU15
Realizes the original function given to the node, such as performing a predetermined cooperative operation while transmitting / receiving data to / from another node.

FIG. 6 shows an example of the internal structure of the network connection device 20. As shown in the drawing, first and second data transmitting / receiving units 21a and 22b are connected to each connection cable 14.
Is connected. These two data transmitting / receiving sections 21a, 21
Like the data transmission / reception unit 12a installed in the node, the optical module 2a is also configured by an optical module having a function of performing optical-electrical conversion.

The data (light) received by the first data transmission / reception unit 21a is converted into an electric signal,
1b and sent to the data transfer unit 25. The data (light) received by the second data transmission / reception unit 22a is converted into an electric signal, and the second data processing unit 21b and the data transfer unit 25
Sent to

In the data transfer unit 25, in addition to the function of shaping the signal, the data received from the first data transmission / reception unit 21a is transferred (through) to the second data transmission / reception unit 22a as it is, and the second data transmission / reception unit 22a Is transferred (through) to the first data transmitting / receiving unit 21a as it is. At this time, the first and second data transmission / reception units 21a, 22
a, the first and second changeover switches 24
a and 24b are provided to allow or stop the above transfer processing. The two changeover switches 24a and 24b used in the present embodiment select one of two inputs (points a and b) and output one of the inputs (point c). Is connected to the output of the data transfer unit 25, and the other input terminal (point b) is connected to the outputs of the first and second data processing units 21b and 22b.

Further, the two changeover switches 24a, 24
Switching of b is controlled by a control signal sent from the information processing device 23. Therefore, by connecting both the changeover switches 24a and 24b to the other input terminal b point side, the first data transmission / reception unit 21a and the second data transmission / reception unit 22a can be disconnected (switch open), and the through operation can be performed. Can be suppressed. Each data transmitted via the connection cable 14 is transmitted to the first and second data transceivers 2.
1a and 22a. Accordingly, a bus network of the digital chain system can be constructed.

On the other hand, the two changeover switches 24a and 24b are
By connecting to the one input terminal point a side, the first data transmission / reception unit 21a and the second data transmission / reception unit 22a can be connected (switch closed) and transferred (through).

The first and second data processing units 21b, 2b
The basic function 2b has a signal conversion function and a data framing function, similar to the data processing unit 12c installed in the node. As a reception mode, it is possible to switch between an address mode in which only a frame addressed to itself is received and a 0 mode in which all frames are received. Further, it receives data (various command commands, etc.) sent from the information processing unit 23, performs signal conversion and framing processing on the data, and sends it to each node (actually, from each of the changeover switches 24a, 24b). Connection cable 14 via each data transmission / reception unit 21a, 21b
To be sent).

In the context of the present invention, the information processing device 23 sends the control signal described above and
It has a function of switching control of the changeover switches 24a and 24b. This control signal is, of course, to select the other input point b when it is desired to construct a normal digital chain bus type network, and to transmit the control signal through the network connection device 20 when it is desired. Is controlled to select one input point a.

It is possible to determine when to output the switching control signal specifically based on, for example, data flowing from the data processing units 21b and 22b and flowing through the network.
As an example, it is possible to determine whether or not a failure has occurred in a part of the network, and when it is determined that a failure has occurred, it is possible to switch to one input unit point a side. For example, when a failure signal is generated from any node, it can be determined whether or not a failure has occurred by receiving the information.

Further, a judgment means for judging the presence or absence of a failure may be provided in the network connection device 20. That is,
Since the frame transmitted from each node always reaches both ends of the network, the first and second communication units 21 and 22
Is taken into the information processing device 23 via the. Therefore, when a frame is received from only one of the communication units, it can be determined that a failure has occurred. Also, instead of being based on the frame transmitted from each node as described above, for example, the information processing device 23 transmits a test frame (data) from one communication unit side, which can be received by the other communication unit. The presence or absence of a failure can be determined based on whether or not there is a failure. In this case, changeover switches 24a and 24b for switching between two inputs
Is provided so that a frame from the information processing apparatus 23 can be transmitted. Of course, other methods may be used.

As an output timing of the switching control signal, for example, there may be a case where it is desired to stop all the nodes 10a to 10d all at once. When it is desired to stop all at once, for example, it is possible to make a determination based on a frame transmitted from some node,
The PU 15 can also make a decision by itself (for example, based on time or the like). In addition, the signal may be based on a signal provided from the outside. In this case, the signal may be received not only from a node connected to the network but also from a source other than the network.

In correspondence with the block diagram shown in FIG. 2, the first data transmitting / receiving section 21a and the first data processing section 2a
1b constitutes the first communication unit 21, and the second data transmission / reception unit 2
2a and the second data processing unit 22b constitute the second communication unit 22. Furthermore, first and second changeover switches 24a and 24b
Constitute the switch 24. The switch 24 may include the data transfer unit 25 as a component. In addition, the first and second changeover switches 24a, 24b
As described above, a switch for selectively selecting and outputting two inputs is used. This is because a frame can be transmitted from the information processing device 23 to each node as described above. The present invention is not necessarily required to have such a configuration, and may be any device that can switch between data through and data discarding, such as a simple ON-OFF switch.

The information processing device 23 has a monitor 26
Is connected, it is possible to display, for example, data collected from each node and to display the operation state of the node.

Further, in the present embodiment, two communication units 21 and
2 is provided. Thereby, both end points of the bus network can be connected to different communication units. The data to be sent to the information processing unit 23 is not the data whose waveform has been shaped by the data transfer unit, but is the data received via the first and second data transmission / reception units 21a and 22a. In other words, in the case of a node, noise or the like may be caused by transmitting (transferring) on a network, and if the data is transferred as it is, data may be distorted and erroneous data may be generated. Therefore, it is necessary to shape the waveform in the data transfer unit. On the other hand, in the case of the network connection device, since the data to be sent to the information processing device 23 is not transferred thereafter, it can be sufficiently analyzed without performing waveform shaping.
Of course, if it is desired to shape the waveform, for example, the signal may be branched from between the output of the data transfer unit 25 and the changeover switches 24a and 24b and sent to the data processing units 21b and 22b, or a processing unit for separately shaping the waveform may be provided. Good.

In the above-described embodiment, the device constituting the node is a PLC, but the node to which the present invention is applied is not limited to this, and it goes without saying that various devices can be used. .

[0056]

As described above, in the network connection device according to the present invention, the network can be looped (pseudo-looped) while maintaining the advantage of high-speed communication of the bus type network. That is, if the received data is not transferred in the network connection device, the bus-type network performs a normal operation and can perform high-speed communication. On the other hand, when the transfer is performed, the network can be looped (pseudo-looped), and various effects can be exhibited.

According to the third aspect of the present invention, even if there is an abnormality such as a failure in a part of the bus type network, the frame transmitted from a certain node can be transferred directly or by transferring the data through the network connection device. Since the data can be transmitted via the inside of the network connection device, it is resistant to failure.

Further, according to the fourth aspect of the present invention, the bus type network in a normal state can be looped, and by causing frame collisions positively, the nodes connected to the network can be simultaneously controlled. Can be stopped. That is, high-speed execution can be achieved by a simple process of switching a switch or the like.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an example of a conventional bus network.

FIG. 2 is a diagram showing an example of a bus type network in which a network connection device according to the present invention is mounted.

FIG. 3 is a diagram showing an example of a bus network in which the network connection device according to the present invention is mounted.

FIG. 4 is a diagram illustrating an example of an internal structure of a node.

FIG. 5 is a diagram illustrating an example of a data structure of a frame flowing on a network.

FIG. 6 is a diagram showing an embodiment of a network connection device according to the present invention.

[Explanation of symbols]

 Reference Signs List 20 network connection device 21 first communication unit 22 second communication unit 23 information processing device 24 switch 24a first changeover switch 24b second changeover switch

Claims (5)

[Claims] 1. A network connection device that can be connected to and disconnected from both end points of a bus network, a first function of discarding data sent from the both end points, and one end point of the bus network. A second function of transferring the data sent from the second function to the other end point, and a control means for switching between the first function and the second function to enable one of the functions. Network connection device. 2. Two communication units respectively connected to both ends of a bus-type network so as to be able to transmit and receive, a switch unit connecting the two communication units, and a control unit controlling a switching operation of the switch unit. A first state in which the two communication units are shut off by switching the switch means, and connecting the two communication units;
A network connection device configured to be in a second state in which data received by one communication unit is transferred to another communication unit and transmitted. 3. A network connection device having two connection points that can be connected to and disconnected from both end points of a bus-type network, wherein the determination unit determines whether the network is normal or abnormal, and the determination unit determines Control means for switching between control for sending data taken from one of the two connection points to the other connection point and control for not sending the data in accordance with the result. Network connection device. 4. A communication control method in a network in which both ends of a bus type network are connected by a network connection device, wherein when the bus type network is normal, received data sent into the network connection device is discarded. When the bus-type network is abnormal, the communication control method is characterized in that the received data sent from one end point into the network connection device is transferred to the other end point. 5. A communication control method in a network in which both end points of a bus type network are connected by a network connection device, wherein in a normal state, received data sent into the network connection device is discarded, and A communication control method characterized by disabling communication between nodes connected to the network by mutually transferring received data sent into a network device.